Conventionally, there has been known an image forming apparatus that performs image formation by electrophotography. This apparatus electrically charges toner particles and performs image formation using electrostatic force. Therefore, when the amount of electrostatic charge of toner particles changes, the density and quality level of an output image changes accordingly. The amount of electrostatic charge of toner particles largely varies with the usage environment, the density of an image to be output, output elapsed time, etc.
Particularly, in an electrophotographic image forming apparatus using a two-component development unit, i.e. in an image forming apparatus that uses toner particles and carrier particles as a developer, image formation is performed by filling up an electrostatic latent image formed by image exposure with electrostatically charged toner particles. In this apparatus, changes in image density due to changes in the amount of electrostatic charge of toner particles are frequently caused. The changes in image density affect not only the density of a solid image, but also all gradations. Therefore, gradation change is caused along with changes in the amount of electrostatic charge of toner.
To reduce variation in density of solid portions and halftone portions, i.e. variation in gradation, gradation correction feedback control is generally widely performed. In this control, an image patch (pattern image) at a desired gradation is output onto an image bearing member or a transfer member, and after measuring patch density (toner amount) on the image bearing member or the transfer member, gradation conversion is performed on the output image such that the output density becomes closer to a target density.
However, the gradation correction control based on the above-mentioned output density of the image patch is feedback control in which a gradation conversion table is created after measuring the density (toner amount) of the image patch, and then gradation conversion processing using the gradation conversion table is applied to an image to be output. Therefore, it is impossible to prevent dead time in control (time delay in control) from being generated. That is, this inevitably causes fluctuation in image density and gradation on a short-term basis.
To solve such a problem, there has been proposed a technique, e.g. in Patent Literature 1, for stabilizing image density by performing feedforward control in which an amount of electrostatic charge of toner particles is estimated (predicted), and a contrast potential in image formation is controlled on a real-time basis.